Notes on opal-bearing surface rocks in the Northern Territory with reference to supplies of aggregate and sand

Geological work in the Territory since 1946 has provided more information on the composition of the so-called "porcelianite" which occupies much of the higher ground in the Peninsula on which the town of Darwin is built. This "porcelianite" is actually a fossil soil horizon and represents silicified lower zones of a laterite profile developed on Lower Cretaceous shales during Tertiary time. It is certain that at least some of the silica cementing the rock is in opaline form. The occurrence, distribution, types, and uses of opaline-material in the Northern Territory are discussed in this report. The results of testing conducted on aggregate samples are recorded, and recommendations are set down for further investigation of opaline-material.